Processing cache miss rates to determine memory space to add to an active cache to reduce a cache miss rate for the active cache

ABSTRACT

Provided are a computer program product, system, and method for processing cache miss rates to determine memory space to add to an active cache to reduce a cache miss rate for the active cache. During caching operations to the active cache, information is gathered on an active cache miss rate based on a rate of access to tracks that are not indicated in the active cache list and a cache demote rate. A determination is made as to whether adding additional memory space to the active cache would result in the active cache miss rate being less than the cache demote rate when the active cache miss rate exceeds the cache demote rate. A message is generated indicating to add the additional memory space when adding the additional memory space would result in the active cache miss rate being less than the cache demote rate.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a computer program product, system, andmethod for processing cache miss rates to determine memory space to addto an active cache to reduce a cache miss rate for the active cache.

2. Description of the Related Art

A cache management system buffers tracks in a storage device recentlyaccessed as a result of read and write operations in a faster accessstorage device, such as memory, than the storage device storing therequested tracks. Subsequent read requests to tracks in the fasteraccess cache memory are returned at a faster rate than returning therequested tracks from the slower access storage, thus reducing readlatency.

A cache management system may maintain a linked list having one entryfor each track stored in the cache, which may comprise write databuffered in cache before writing to the storage device or read data. Inthe commonly used Least Recently Used (LRU) cache technique, if a trackin the cache is accessed, i.e., a cache “hit”, then the entry in the LRUlist for the accessed track is moved to a Most Recently Used (MRU) endof the list. If the requested track is not in the cache, i.e., a cachemiss, then the track in the cache whose entry is at the LRU end of thelist may be removed (or destaged back to storage) and an entry for thetrack data staged into cache from the storage is added to the MRU end ofthe LRU list. With this LRU cache technique, tracks that are morefrequently accessed are likely to remain in cache, while data lessfrequently accessed will more likely be removed from the LRU end of thelist to make room in cache for newly accessed tracks.

There is a need for improved techniques for determining when to addmemory space to an active cache of a computing system.

SUMMARY

Provided are a computer program product, system, and method forprocessing cache miss rates to determine memory space to add to anactive cache to reduce a cache miss rate for the active cache. Duringcaching operations to the active cache, information is gathered on anactive cache miss rate based on a rate of access to tracks that are notindicated in the active cache list over time and a cache demote ratebased on a rate of an amount of data removed from the active cache overtime. A determination is made as to whether the active cache miss rateexceeds the cache demote rate. A determination is made as to whetheradding additional memory space to the active cache would result in theactive cache miss rate being less than the cache demote rate in responseto determining that the active cache miss rate exceeds the cache demoterate. A message is generated to a user of the computer system indicatingto add the additional memory space to the active cache in response todetermining that adding the additional memory space would result in theactive cache miss rate being less than the cache demote rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a computing environment.

FIG. 2 illustrates an embodiment of a Least Recently Used (LRU) list.

FIG. 3 illustrates an embodiment of a cache control block.

FIG. 4 illustrates an embodiment of a ghost cache control block.

FIG. 5 illustrates an embodiment of time interval cache accessstatistics gathered over a time interval.

FIG. 6 illustrates an embodiment of calculated miss rates based on thetime interval cache statistics.

FIG. 7 illustrates an embodiment of operations to initiate gathering ofcache statistics.

FIGS. 8A and 8B illustrate an embodiment of an operation to process aread request to a track.

FIG. 9 illustrates an embodiment of operations to gather and analyzecache access statistics.

FIG. 10 illustrates an embodiment of operations to provide notificationto add memory space to the active cache.

FIG. 11 illustrates an embodiment of operations to add memory space tothe active cache.

FIG. 12 illustrates a computing environment in which the components ofFIG. 1 may be implemented.

DETAILED DESCRIPTION

A storage controller demotes tracks from an active cache to make room tocache for data for I/O requests, e.g., read or write requests. If thedata for a read request is already in the cache, i.e., a cache hit, thenthe requested data does not need to be staged into the cache. If thedata for the read request is not in the cache, i.e., a cache miss, or ifthere is write data for the request, then tracks in the cache may needto be demoted or removed from the cache to make space for the read datato stage into cache or the write data. Tracks are demoted based oncaching algorithms. If the cache miss rate exceeds the cache demoterate, then the rate at which read data needs to be added to cacheexceeds the rate at which data is demoted from cache, and then new I/Orequests will need to be queued until space is freed in the cache. Suchqueuing of I/O requests can cause severe performance problems and highresponse times to the queued I/O requests.

Described embodiments provide technological solutions to cache computertechnology to determine whether adding memory space to the cache couldreduce the cache miss rate to be less than the cache demote rate andthen send a message to the user indicating to add additional memoryspace. Further, the computer system may maintain unused cachepartitions, referred to as ghost caches, and then calculate ghost cachemiss rates for different combinations of unused cache partitions orghost caches. The message to the user may indicate to immediately addthe unused memory partitions, or ghost cache space, to the active cacheto lower the cache miss rate to avoid queuing of I/O requests.

Described embodiments provide improvements and technological solutionsto the problem in cache computer technology where cache miss ratesexceed cache demote rates resulting in I/O request queuing. Thedescribed solutions determine the effect of adding unused memory spaceto the active cache to reduce the cache miss rate to be below the cachedemote rate so that I/O requests are not queued. The technologicalsolutions of the described embodiments monitor cache miss and cachedemote rates during time intervals to determine whether to signal theuser to add unused cache space, or ghost cache space, to the activecache to avoid the queueing of I/O requests.

FIG. 1 illustrates an embodiment of a computing environment. A computingsystem 100 accesses data in volumes 102 (e.g., Logical Unit Numbers,Logical Devices, Logical Subsystems, etc.) configured in a storage 104.The computing system 100 includes a processor 106 and a memory 108,including an active cache 110 to cache data for the storage 104. Theprocessor 106 may comprise one or more central processing units (CPUs)or a group of multiple cores on a single CPU. The active cache 110buffers data requested by processes within the computing system.Alternatively, the computing system 100 may comprise a storagecontroller that processes Input/Output (I/O) access requests for tracksin the storage 104 from hosts 118 connecting to the computing system 100(storage controller) over a network 120.

The memory 108 may further include a plurality of memory spacepartitions that are not available for use in the computer system 100referred to as ghost caches 112 ₁, 112 ₂ . . . 112 _(N). Each ghostcache 112 ₁, 112 ₂ . . . 112 _(N) comprises a reserved partition of thememory space 108 that has not been allocated to the active cache 110 andmay be allocated to the active cache 110 to increase the amount ofmemory space in the active cache 110. The unused memory space partitionfor each ghost cache 112 ₁, 112 ₂ . . . 112 _(N) may be separately andindependently added to the active cache 110.

The memory 108 further includes a storage manager 122, cache manager124, and cache provisioning service 126. The storage manager 122 managesaccess requests from internal processes in the computing system 100and/or from hosts 118 for tracks in the storage 104. The cache manager124 maintains accessed tracks in the active cache 110 for future readaccess to the tracks to allow the accessed tracks to be returned fromthe faster access cache 110 instead of having to retrieve from thestorage 104. A track may comprise any unit of data configured in thestorage 104, such as a track, Logical Block Address (LBA), etc., whichis part of a larger grouping of tracks, such as a volume, logicaldevice, etc. The cache provisioning service 126 may manage adding memoryspace partitions for one or more of the ghost caches 112 ₁, 112 ₂ . . .112 _(N) to the active cache 110, such as if the user of the computersystem 100 pays a fee to a service provider of the computer system 100for additional memory space. In this way, partitions of the memory 108designated as ghost caches 112 ₁, 112 ₂ . . . 112 _(N) that areinitially configured as reserved but unavailable in the computer system100, can later be activated by the cache provisioning service 126 to addto the active cache 110 upon user payment of a fee when additionalmemory space is needed in the cache 110. In one embodiment, the cacheprovisioning service 126 can solicit payment information from the userto add one or more of the unused partitions of the ghost caches 112 ₁,112 ₂ . . . 112 _(N). Alternatively, the user of the computer system 100may interact with an Internet based web service to purchase additionalmemory partitions, and the remote Internet based web service may signalthe cache provisioning service 126 to add purchased partitions of memoryof the ghost caches 112 ₁, 112 ₂ . . . 112 _(N) to the active cache 110.

The cache manager 124 maintains cache management information 130 in thememory 108 to manage read (unmodified) and write (modified) tracks inthe cache 110. The cache management information 130 may include a trackindex 132 providing an index of tracks in the cache 110 to cache controlblocks in a control block directory 300; an active cache Least RecentlyUsed (LRU) list 200 _(A) for tracks in the cache 110; and one or moreghost cache LRU lists 200 ₁ . . . 200 _(N) to indicate tracks that couldhave been stored in the cache 110 if additional memory space was addedto the cache to store the tracks indicated in the ghost cache LRU lists200 ₁ . . . 200 _(N). Each ghost cache LRU list 200 ₁ . . . 200 _(N) maybe associated with one of the ghost caches 112 ₁, 112 ₂ . . . 112 _(N).However, tracks indicated in the ghost cache LRU lists 200 ₁ . . . 200_(N) are not stored in the ghost caches 112 ₁, 112 ₂ . . . 112 _(N) orany other part of the memory 108. The control block directory 300includes the cache control blocks, where there is one cache controlblock for each track in the cache 110 providing metadata on the track inthe cache 110. The track index 132 associates tracks with the cachecontrol blocks providing information on the tracks in the cache 110.Upon determining that the active cache LRU list 200 _(A) is full or hasreached a threshold level, the active cache LRU list 200 _(A) is used todetermine tracks to demote.

In one embodiment, there may only be cache control blocks 300 for thetracks in the cache 110 and no cache control blocks are allocated fortracks indicated in the ghost cache LRU lists 200 ₁ . . . 200 _(N). Atrack indicated in the ghost cache LRU lists 200 ₁ . . . 200 _(N) is notstored in the cache 110. In an alternative embodiment, the cache controlblock directory 300 may maintain ghost cache control blocks for tracksindicated in the ghost cache LRU lists 200 ₁ . . . 200 _(N) even thoughthey are not stored in the cache 110. In one such embodiment using ghostcache control blocks for tracks indicated in the ghost cache LRU lists200 ₁ . . . 200 _(N), the cache control blocks used for tracks in theghost cache LRU lists 200 ₁ . . . 200 _(N), may be the same as thoseused for tracks in the cache 110, such as shown for FIG. 3, withoutinformation specific to the track being maintained in the cache 110. Ina still further alternative embodiment, the cache control blocks usedfor tracks in the ghost cache LRU lists 200 ₁ . . . 200 _(N) may havefewer fields and less information than those for tracks maintained inthe cache 110, such as shown in FIG. 4. In such embodiments, ghost cachecontrol blocks for tracks indicated in the ghost cache LRU lists 200 ₁ .. . 200 _(N) may only maintain information about the ghost cache LRUlist 200 ₁ . . . 200 _(N) where the track is indicated. For instance,for a track in the cache 110, a cache control block may require 256bytes for a 4K block, but a ghost cache control block for a track onlyindicated in one of the ghost cache LRU lists 200 ₁ . . . 200 _(N) mayonly comprise 20 bytes.

During caching operations, the cache manager 124 may gather for specificintervals of time, such as 15 second intervals, cache access statistics500 on access to tracks in the cache 110 indicated in the active cacheLRU list 200 _(A) and tracks indicated in the ghost cache LRU lists 200₁ . . . 200 _(N), which are not stored in the cache 110 or the ghostcaches 112 ₁, 112 ₂ . . . 112 _(N). The cache manager 124 may furthercalculate cache miss rates 600 for the currently used cache 110 and fora hypothetical expanded cache having space to store tracks indicated inone or more ghost cache LRU cache lists 200 ₁ . . . 200 _(N). Forinstance, if there are N tiers or levels of ghost caches that are beingconsidered, then there are N ghost cache LRU lists 200 ₁ . . . 200 _(N)and N instances of ghost cache miss rates may be calculated. The ghostcache miss rate i for ghost caches 112 _(i) for i=1 to N is based on thecache miss rate if the active cache 110 and all ghost caches 112 ₁ . . .112 i were available to cache tracks. Each of the N ghost cache LRUlists 200 ₁ . . . 200 _(N) may have the same number of track entries.This allows one to compare how the cache miss rate would improve ifdifferent amounts of memory or the ghost caches 112 ₁, 112 ₂ . . . 112_(N) are added to the cache 110. For instance, each ghost cache levelmay add a fixed amount of memory space, where each increasing level imay add i times the fixed amount of memory space. Alternatively, thelevels may correspond to variable amounts of memory space to be added.

Providing information on cache miss rates for adding different amountsof memory informs administrators on specific improvements that could berealized for adding specific amounts of memory.

In the described embodiments, the lists 200 _(A) and 200 ₁ . . . 200_(N) comprise LRU lists. In alternative embodiments, the lists 200 _(A)and 200 ₁ . . . 200 _(N) may comprise other types of lists to organizeindication of tracks in the cache 110.

The storage manager 122, cache manager 124, and cache provisioningservice 126 are shown in FIG. 1 as program code loaded into the memory108 and executed by the processor 106. Alternatively, some or all of thefunctions may be implemented in hardware devices in the storagecontroller 104, such as in Application Specific Integrated Circuits(ASICs).

The storage 104 may comprise one or more storage devices known in theart, such as a solid state storage device (SSD) comprised of solid stateelectronics, EEPROM (Electrically Erasable Programmable Read-OnlyMemory), flash memory, flash disk, Random Access Memory (RAM) drive,storage-class memory (SCM), Phase Change Memory (PCM), resistive randomaccess memory (RRAM), spin transfer torque memory (STM-RAM), conductivebridging RAM (CBRAM), magnetic hard disk drive, optical disk, tape, etc.The storage devices may further be configured into an array of devices,such as Just a Bunch of Disks (JBOD), Direct Access Storage Device(DASD), Redundant Array of Independent Disks (RAID) array,virtualization device, etc. Further, the storage devices may compriseheterogeneous storage devices from different vendors or from the samevendor.

The memory 108 may comprise a suitable volatile or non-volatile memorydevices, including those described above.

The network 120 may comprise a Storage Area Network (SAN), a Local AreaNetwork (LAN), a Wide Area Network (WAN), the Internet, and Intranet,etc.

FIG. 2 illustrates an embodiment of one of the LRU lists 200 _(i), suchas LRU cache lists 200 _(A) and 200 ₁ . . . 200 _(N), as having a mostrecently used (MRU) end 202 identifying a track most recently added tothe cache 110 or most recently accessed in the cache 110 and a leastrecently used (LRU) end 204 from which the track identified at the LRUend 204 is selected to demote from the cache 110. The LRU end 204 pointsto a track identifier, such as a track identifier address or a cachecontrol block index for the track, of the track that has been in thecache 110 the longest for tracks indicated in that list 200 _(A). Theghost cache LRU lists 200 ₁ . . . 200 _(N) identify tracks that couldhave been stored in the cache if the corresponding memory space wasadded to the active cache 110 to store the tracks indicated in the ghostcache LRU lists 200 ₁ . . . 200 _(N).

FIG. 3 illustrates an embodiment of a cache control block 300 _(i) forone of the tracks in the active cache 110, including, but not limitedto, a cache control block identifier 302, such as an index value of thecache control block 300 _(i); a track ID 304 of the track in the storage104; the cache LRU list 306 in which the cache control block 300 _(i) isindicated; an LRU list entry 308 at which the track is indicated; acache timestamp 310 indicating a time the track was added to the cache110 and indicated on the LRU list 304; and additional track metadata 312typically maintained for tracks stored in the cache 110.

FIG. 4 illustrates an embodiment of a ghost cache control block 400 _(i)that may be maintained for a track indicated in the ghost cache LRUlists 200 ₁ . . . 200 _(N), but not stored in the cache 110. The ghostcache control block 400 _(i) includes fields 402, 404, 406, and 408having the same type of information in fields 302, 304, 306, 308,respectively, in the cache control block 300 _(i), but providinginformation on the ghost cache LRU list 406 and an entry 408 in theghost cache LRU list entry for the track. The cache control blockdirectory 300 may include cache 300 _(i) and ghost 400 _(i) cachecontrol blocks.

FIG. 5 illustrates an instance of time interval cache statistics 500_(i) gathered for one time interval. The cache manager 124 may gathercache access statistics for time intervals within a large time periodduring caching operations. The time interval cache access statistics 500_(i) identify the time interval 502 for which the information isgathered; active cache misses in the time interval 504 for the activecache 110, where a cache miss comprises a read to a track in a volume102 that is not in the active cache 110; ghost cache 112 _(i) cachemisses for the interval 506 i, for i=1 . . . N, indicating a number ofcache misses if ghost caches 112 ₁ . . . 112 _(i) were added to theactive cache 110; and an amount of data demoted 508 from the activecache 110 during the time interval 502.

FIG. 6 illustrates an embodiment of calculated miss rates 600 calculatedfrom the gathered access statistics 500 _(i) including a peak activecache miss rate 602 comprising a highest active cache miss rate for thetime intervals 500 _(i), where the active cache miss rate is calculatedby dividing the number of cache misses 504 in the time interval by anamount of time in the time interval and then multiplying that quotientby an average Input/Output (“I/O”) size. The peak active cache miss rate602 may comprise the cache miss rate calculated from the greatest activecache misses 504 from the maintained time interval cache statistics 500_(i). The access statistics 600 further include peak ghost cache missrates 604 ₁ . . . 604 _(N) calculated for each of the ghost caches 112₁, 112 ₂ . . . 112 _(N), where each peak ghost cache miss rate 604 _(i)may be calculated by dividing the highest ghost cache misses 506 _(i)for all time intervals divided by the time interval, and thenmultiplying that division result by the average I/O size. A cache demoterate 606 comprises the amount of data demoted from the cache 508 for alltime intervals divided by a total amount of time of all the consideredthe time intervals. For instance, if there are twenty 15 secondintervals stored, then the total amount of time of the time intervalscomprises 5 minutes.

FIG. 7 illustrates an embodiment of operations performed by the cachemanager 124 to generate a current time interval cache access statistics500 _(i). Upon initiating (at block 700) the gathering of cache accessstatistics, a variable i is set (at block 702) to 1. Time interval cachestatistics 500 _(t) for time interval t are generated (at block 704) orcleared if there is already an entry 500 _(t) for the time interval t.When (at block 706) the current time interval t expires, if (at block708) t is not the last time interval in a time period, then t isincremented (at block 710) by one and control proceeds back to block704. If the time interval t is the last time interval in a time period,then control proceeds back to block 702 to reset the time interval tothe first time interval, and then sequence through the time intervals inthe time period.

FIGS. 8a and 8b illustrate an embodiment of operations performed by thecache manager 124 to process a read request to a track in the storage104. Upon receiving (at block 800) a read request to a track, if (atblock 802) the track is indicated in the active cache LRU list 200 _(A),then the track is returned (at block 804) to the request from the activecache 110 and the indication of the accessed track is moved (at block806) to the MRU end 202 of the active cache LRU list 200 _(A), such asby adjusting the LRU list entries 308, or LRU pointers, for the accessedtrack in the active cache LRU list 200 _(A).

If (at block 802) the track is not indicated in the active cache LRUlist 200 _(A), then the track is accessed (at block 808) from thestorage 104 to return to the request. The cache manager 124 increments(at block 810) the active cache misses 504 in the cache accessstatistics 500 _(t) for the current time interval t. If (at block 812)the track is in a ghost cache LRU list 200 _(i) of the ghost cache LRUlists 200 ₁ . . . 200 _(N), then the cache manager 124 removes (at block814) indication of the accessed track from the ghost cache LRU list 200_(i). The cache manager 124 increments (at block 816) the ghost cachemisses 506 ₁, 506 ₂ . . . 506 _(i−1) in the cache access statistics 500_(t) for the current interval for all ghost caches 112 ₁, 112 ₂ . . .112 _(i−1) prior to the ghost cache 112 _(i) whose ghost cache LRU list200 ₁ indicates the accessed track. These indications are made toindicate that the track was missed in all ghost caches LRU lists 200 ₁ .. . 200 _(i−1) up to the ghost cache LRU list 200 _(i) indicating theaccessed track because a cache size corresponding to the size of theactive cache 200 _(A) and all ghost caches 112 ₁ . . . 112 _(i−1) wasnot large enough to include the accessed track. If (at block 812) thetrack is not any of the ghost cache LRU list 200 ₁ . . . 200 _(N), thenthe cache manager 124 increments (at block 818) the ghost cache misses506 ₁, 506 ₂ . . . 506 _(N) in the cache access statistics 500 _(t) forthe current time interval t for all ghost caches 112 ₁, 112 ₂ . . . 112_(N).

From block 816 or 818, after incrementing the ghost cache read misses506 ₁, 506 ₂ . . . 506 _(N) for some or all of the ghost caches, if (atblock 820) the active cache LRU list 200 _(A) is full, then the track atthe LRU end 204 of the active cache LRU list 200 _(A) is demoted (atblock 822) and discarded from the active cache 110. The amount of data(track) demoted is added (at block 824) to the amount of data demoted508 field for the cache statistics 500 _(t) for the current timeinterval t. If (at block 826) the first ghost cache LRU list 200 ₁ isnot full, then the indication of the track demoted from the active cache110 is added (at block 830) from the LRU end 204 of the active cache LRUlist 200 _(A) to the MRU 202 of the first ghost cache LRU list 200 ₁.From block 830 or if (at block 820) the active cache LRU list 200 _(A)is not full, the cache manager 122 adds (at block 832) the accessedtrack (from the active 200 _(A) or ghost cache 200 ₁ LRU list) to theMRU end 202 of the cache LRU list 200 _(A).

If (at block 826) the first ghost cache LRU list 200 ₁ is full, thencontrol proceeds to block 828 to adjust the ghost cache LRU lists 200 ₁. . . 200 _(N) by demoting (removing) indication of a track from the LRUend 204 of one ghost cache LRU list 200 i and adding the demoted trackto the MRU end 202 of a next ghost cache LRU list 200 _(i+1), until thelast Nth ghost cache LRU list 200 _(N) is reached, such that the trackdemoted from the LRU end 204 of the last ghost cache LRU list 200 _(N)is demoted without adding to a further ghost cache LRU list. Controlproceeds (at bock 828) to block 834 in FIG. 8b to move a track throughthe ghost cache LRU lists 200 ₁ . . . 200 _(N) when room needs to bemade for a track demoted from the cache LRU list 200 _(A). At block 834,the cache manager 124 demotes (at block 834) the track at the LRU end204 of the first ghost cache LRU list 200 ₁. A variable i is set (atblock 836) to two for the next ghost cache LRU list 200 ₂ to start thecascade of demotions of tracks through the ghost cache LRU lists 200 ₁ .. . 200 _(N). If (at block 838) the ghost cache LRU list 200 _(i) isfull, then the cache manager 124 demotes (removes) (at block 840) atrack at the LRU end 204 of the ghost cache LRU list 200 _(i) and adds(at block 842) the track demoted from the ghost cache LRU list 200_(i−1) to the MRU end 202 of the ghost cache LRU list 200 _(i). If (atblock 844) i is not equal to N, i.e., there is another ghost cache LRUlist 200 _(i+1) to which a track demoted from the last considered ghostcache LRU list 200 _(i) can be added, then i is incremented (at block846) and control proceeds back to block 738 to move the track demotedfrom the last considered ghost cache LRU list to a further ghost cacheLRU list.

If (at block 838) the ghost cache LRU list 200 _(i) is not full, thenthe track demoted from the ghost cache LRU list 200 _(i−1) is added (atblock 848) to the MRU end 202 of the ghost cache LRU list 200 _(i). Fromblock 848 or if (at block 844) i is N, i.e., the last ghost cache LRUlist 200 _(N) was processed, then control proceeds (at block 850) backto block 828 in FIG. 8 a.

With the described operations of FIGS. 8a and 8b , a track is demotedfrom one of the ghost cache LRU lists 200 _(i) to make room to add atrack demoted from the active cache 110 or from a previous ghost cacheLRU list 200 _(i−1), so that a track demoted from the active cache 110moves sequentially through ghost cache LRU lists 200 ₁ through 200 _(N)to simulate how the track would be managed if the cache had additionalmemory to accommodate tracks indicated in the ghost cache LRU lists 200₁ . . . 200 _(N).

When adding a track to the cache 110, the cache control block 300 _(i)for that address in the cache 110 would be updated to includeinformation on the track and the entry in the cache LRU list 200 _(A)indicating the track. Further, the track index 132 would be updated toinclude the cache control block index for the track in the cache 110. Inone embodiment, if a track is indicated in a ghost cache LRU list 200_(i), there may be no cache control block created for the track andindicated in the cache control block directory 300. In an alternativeembodiment, a full cache control block 300 _(i) may be created in thecache control block directory 300 for the track indicated in the ghostcache LRU list 200 _(i). In a still further alternative embodiment, asmaller ghost cache control block 400 _(i) may be added to the cachecontrol block directory 300 for the track indicated in the ghost cacheLRU list 200 _(i) having minimal information to identify the track inone of the ghost cache LRU lists 200 _(i). When a track is moved fromone ghost cache LRU list 200 _(i) to a next ghost cache LRU list 200_(i+1), then the information in the cache control block, such as in theghost cache LRU list 406 and LRU list entry 408, for that track would beupdated to indicate the new location in the new ghost cache LRU list 406to which the track is added. The cache control block index for the ghostcache control block 400 _(i) may be added to the track index 132 for thetrack indicated in the ghost cache LRU list 200 _(i).

With the described embodiments of FIGS. 8a and 8b , tracks demoted fromthe active cache 110 and active cache LRU list 200 _(A) are added to theghost cache LRU list 200 _(i) where accesses to the track once removedfrom the active cache 110 can be tracked as if the track is in theactive cache 110. Accesses to a track indicated on the ghost cache LRUlists 200 ₁ . . . 200 _(N) can be used to determine the cache miss rateof the active cache 110 and ghost caches 112 ₁, 112 ₂ . . . 112 _(N)that would be realized if the cache 110 was expanded to include thespace of one or more of the ghost caches 112 ₁, 112 ₂ . . . 112 _(N) toaccommodate the tracks indicate in the ghost cache LRU lists 200 ₁ . . .200 _(N).

FIG. 9 illustrates an embodiment of operations performed by the cachemanager 124 (or some other component) to periodically initiateoperations to gather and analyze cache access statistics. Uponinitiating (at block 900) such operations, the cache manager 124determines (at block 902) the highest active cache misses 504 for allmaintained time interval cache access statistics 500 _(i). The cachemanager 124 calculates (at block 904) a peak active cache rate 504 asthe determined highest active cache misses 504 _(H) across timeintervals divided by a time of the time interval, and then multiplyingthe quotient is multiplied by an average I/O size. The average I/O sizemay be continually determined by the cache manager 124 based on size ofI/O accesses. The cache manager 124 further determines (at block 906) ahighest ghost cache miss rate for each of the ghost caches 112 ₁, 112 ₂. . . 112 _(N) across the cache access statistics 500 ₁ . . . 500 _(T)across all time intervals T. For each ghost cache 112 _(i), the cachemanager 124 calculates (at block 908) a peak ghost cache miss rate 604_(i) as the determined highest ghost cache misses 506 _(i) across timeintervals divided by a time of the time interval, and this quotient ismultiplied by the average I/O size. The cache demote rate 606 iscalculated by dividing the total amount of data demoted for allmaintained time interval cache access statistics 500 by a total time ofall time intervals for which cache access statistics are considered.

With the described embodiments, active and ghost cache miss rates arecalculated based on actual cache miss rates to the active cache anddeterminations as to whether cache misses would have occurred had ghostcache space been added to the active cache space.

In described embodiments, peaks miss rates are calculated for the activeand ghost caches. In alternative embodiment, the calculated miss ratesfor active and ghost caches may comprise another calculated value basedon time interval cache statistics 500, such as an average, mean, etc.

FIG. 10 illustrates an embodiment of operations performed by the cacheprovisioning service 126 to determine whether to notify the user to addmemory space to the active cache 110. Upon initiating (at block 1000) anoperation to add memory space partitions for the ghost caches 112 ₁, 112₂ . . . 112 _(N) to the active cache 110, the cache provisioning service126 determines (at block 1002) whether the peak active cache miss rate602 exceeds the cache demote rate 606. If not, control ends. Otherwise,if (at block 1002) the cache demote rate 606 is exceeded, then avariable i is set (at block 1004) to one and a determination is made (atblock 1006) whether the peak ghost cache miss rate 604 _(i) for the ithghost cache 112 _(i) exceeds the cache demote rate 606. If not, then thecache provisioning service 126 generates (at block 1008) a message to auser indicating to add an amount of memory space comprising the memorysize of ghost caches 112 ₁, 112 ₂ through 112 _(i). The message may besent via email, text message, or displayed on a display monitor of thecomputer system 100. If (at block 1006) the cache demote rate isexceeded and if (at block 1010) the ghost cache 112 _(i) considered isthe last ghost cache list 112 _(N), then the cache provisioning service126 generates (at block 1012) a message to a user indicating to add anentire new computer system 100, such as a storage controller, becausethe remaining ghost cache 112 ₁, 112 ₂ . . . 112 _(N) partitions arecumulatively not of sufficient size to reduce the cache miss rate belowthe cache demote rate 606. If (at block 1010) ghost cache 112 _(i) doesnot comprise the last ghost cache 112 _(N), then the variable i isincremented by one (at block 1014) and control proceeds back to block1006 to consider whether adding a next incremental memory space of thenext ghost cache 112 _(i+1) would reduce the cache miss rate below thecache demote rate 606.

FIG. 11 illustrates an embodiment of operations performed by the cacheprovisioning service 126 to add an amount of space for ghost caches 112₁, 112 ₂ . . . 112 _(i). The cache provisioning service 126 may interactdirectly with the user through the computer system 100 to solicit andreceive payment to add additional memory space partitions for one ormore of the ghost caches 112 ₁, 112 ₂ . . . 112 _(N). Alternatively, thecache provisioning service 126 may receive an instruction to addadditional memory space from a remote server that handles thetransaction with the customer to add memory space, which may involve theuser of the computer system 100 purchasing the additional memory space.Upon receiving (at block 1100) indication to add memory space partitionsfor ghost caches 112 ₁ through 112 _(i), the cache provisioning service126 would issue the instruction to add memory space partitions for theone or more ghost caches 112 ₁, 112 ₂ . . . 112 _(N) for which paymentwas received from the user. Alternatively, the user of the computersystem 100 may interact with an Internet based web service to purchaseadditional memory partitions, and the remote Internet based webs servicewould send the cache provisioning service 126 the instruction to addpartitions of memory of the ghost caches 112 ₁, 112 ₂ . . . 112 _(N) tothe active cache 110. Upon receiving the instruction, the cacheprovisioning service 126 adds (at block 1102) memory spaces for ghostcaches 112 ₁ through 112 _(i) to the active cache 110 to be available tocache data. To accomplish this, the cache provisioning service 126 mayincrease (at block 1104) the active cache LRU list 200 _(A) to includeentries to accommodate caching to the memory space added to the activecache 110. Information on the provisioned ghost caches 112 ₁ through 112_(i) is removed (at block 1106) from consideration and further generatedtime interval cache access statistics 500 _(i) and the calculated missrates 600.

With the described embodiments, ghost cache miss rates 604 _(i) may becalculated to provide information on an expected cache miss rate wouldif unused memory space for ghost caches 1 through i is allocated to theactive cache 110 and available for use. When the active cache miss ratefalls below the cache demote rate, then I/O requests may experiencesignificant delays to wait for space in the cache to be demoted to makeroom for further I/O requests. Described embodiments determine whetheradding partitions of memory space designated as ghost caches wouldimprove the cache miss rate to fall below the cache demote rate. Ifadding memory partitions for one or more of the ghost caches wouldimprove the cache miss rate to avoid exceeding the cache demote rate,then the user may be given the option to add the memory space partitionsfor one or more of the ghost caches to the active cache 110 to improvethe cache miss rate to an acceptable level.

In the described embodiment, variables “i”, “t”, etc., when used withdifferent elements may denote a same or different instance of thatelement.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The computational components of FIG. 1, including the hosts 118 andcomputing system 100 may be implemented in one or more computer systems,such as the computer system 1202 shown in FIG. 12. Computersystem/server 1202 may be described in the general context of computersystem executable instructions, such as program modules, being executedby a computer system. Generally, program modules may include routines,programs, objects, components, logic, data structures, and so on thatperform particular tasks or implement particular abstract data types.Computer system/server 1202 may be practiced in distributed cloudcomputing environments where tasks are performed by remote processingdevices that are linked through a communications network. In adistributed cloud computing environment, program modules may be locatedin both local and remote computer system storage media including memorystorage devices.

As shown in FIG. 12, the computer system/server 1202 is shown in theform of a general-purpose computing device. The components of computersystem/server 1202 may include, but are not limited to, one or moreprocessors or processing units 1204, a system memory 1206, and a bus1208 that couples various system components including system memory 1206to processor 1204. Bus 1208 represents one or more of any of severaltypes of bus structures, including a memory bus or memory controller, aperipheral bus, an accelerated graphics port, and a processor or localbus using any of a variety of bus architectures. By way of example, andnot limitation, such architectures include Industry StandardArchitecture (ISA) bus, Micro Channel Architecture (MCA) bus, EnhancedISA (EISA) bus, Video Electronics Standards Association (VESA) localbus, and Peripheral Component Interconnects (PCI) bus.

Computer system/server 1202 typically includes a variety of computersystem readable media. Such media may be any available media that isaccessible by computer system/server 1202, and it includes both volatileand non-volatile media, removable and non-removable media.

System memory 1206 can include computer system readable media in theform of volatile memory, such as random access memory (RAM) 1210 and/orcache memory 1212. Computer system/server 1202 may further include otherremovable/non-removable, volatile/non-volatile computer system storagemedia. By way of example only, storage system 1213 can be provided forreading from and writing to a non-removable, non-volatile magnetic media(not shown and typically called a “hard drive”). Although not shown, amagnetic disk drive for reading from and writing to a removable,non-volatile magnetic disk (e.g., a “floppy disk”), and an optical diskdrive for reading from or writing to a removable, non-volatile opticaldisk such as a CD-ROM, DVD-ROM or other optical media can be provided.In such instances, each can be connected to bus 1208 by one or more datamedia interfaces. As will be further depicted and described below,memory 1206 may include at least one program product having a set (e.g.,at least one) of program modules that are configured to carry out thefunctions of embodiments of the invention.

Program/utility 1214, having a set (at least one) of program modules1216, may be stored in memory 1206 by way of example, and notlimitation, as well as an operating system, one or more applicationprograms, other program modules, and program data. Each of the operatingsystem, one or more application programs, other program modules, andprogram data or some combination thereof, may include an implementationof a networking environment. The components of the computer 1202 may beimplemented as program modules 1216 which generally carry out thefunctions and/or methodologies of embodiments of the invention asdescribed herein. The systems of FIG. 1 may be implemented in one ormore computer systems 1202, where if they are implemented in multiplecomputer systems 1202, then the computer systems may communicate over anetwork.

Computer system/server 1202 may also communicate with one or moreexternal devices 1218 such as a keyboard, a pointing device, a display1220, etc.; one or more devices that enable a user to interact withcomputer system/server 1202; and/or any devices (e.g., network card,modem, etc.) that enable computer system/server 1202 to communicate withone or more other computing devices. Such communication can occur viaInput/Output (I/O) interfaces 1222. Still yet, computer system/server1202 can communicate with one or more networks such as a local areanetwork (LAN), a general wide area network (WAN), and/or a publicnetwork (e.g., the Internet) via network adapter 1224. As depicted,network adapter 1224 communicates with the other components of computersystem/server 1202 via bus 1208. It should be understood that althoughnot shown, other hardware and/or software components could be used inconjunction with computer system/server 1202. Examples, include, but arenot limited to: microcode, device drivers, redundant processing units,external disk drive arrays, RAID systems, tape drives, and data archivalstorage systems, etc.

The terms “an embodiment”, “embodiment”, “embodiments”, “theembodiment”, “the embodiments”, “one or more embodiments”, “someembodiments”, and “one embodiment” mean “one or more (but not all)embodiments of the present invention(s)” unless expressly specifiedotherwise.

The terms “including”, “comprising”, “having” and variations thereofmean “including but not limited to”, unless expressly specifiedotherwise.

The enumerated listing of items does not imply that any or all of theitems are mutually exclusive, unless expressly specified otherwise.

The terms “a”, “an” and “the” mean “one or more”, unless expresslyspecified otherwise.

Devices that are in communication with each other need not be incontinuous communication with each other, unless expressly specifiedotherwise. In addition, devices that are in communication with eachother may communicate directly or indirectly through one or moreintermediaries.

A description of an embodiment with several components in communicationwith each other does not imply that all such components are required. Onthe contrary a variety of optional components are described toillustrate the wide variety of possible embodiments of the presentinvention.

When a single device or article is described herein, it will be readilyapparent that more than one device/article (whether or not theycooperate) may be used in place of a single device/article. Similarly,where more than one device or article is described herein (whether ornot they cooperate), it will be readily apparent that a singledevice/article may be used in place of the more than one device orarticle or a different number of devices/articles may be used instead ofthe shown number of devices or programs. The functionality and/or thefeatures of a device may be alternatively embodied by one or more otherdevices which are not explicitly described as having suchfunctionality/features. Thus, other embodiments of the present inventionneed not include the device itself.

The foregoing description of various embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Many modifications and variations are possible in lightof the above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto. The above specification, examples and data provide acomplete description of the manufacture and use of the composition ofthe invention. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims herein after appended.

What is claimed is:
 1. A computer program product for managing an activecache in a computer system to cache tracks stored in a storage, thecomputer program product comprising a computer readable storage mediumhaving computer readable program code embodied therein that whenexecuted performs operations, the operations comprising: indicatingtracks in the active cache in an active cache list; during cachingoperations to the active cache, gathering information on an active cachemiss rate based on a rate of access to tracks that are not indicated inthe active cache list over time and a cache demote rate based on a rateof an amount of data removed from the active cache over time;determining whether the active cache miss rate exceeds the cache demoterate; determining whether adding additional memory space to the activecache would result in the active cache miss rate being less than thecache demote rate in response to determining that the active cache missrate exceeds the cache demote rate; and generating a message to a userof the computer system indicating to add the additional memory space tothe active cache in response to determining that adding the additionalmemory space would result in the active cache miss rate being less thanthe cache demote rate.
 2. The computer program product of claim 1,wherein the gathering information on the active cache miss ratecomprises: for each time interval of a plurality of time intervals,indicating a number of active cache misses that occurred during the timeinterval; determining a number of the active cache misses per unit oftime based on the number of active cache misses that occurred duringeach of the time intervals; and multiplying the determined number of theactive cache misses per unit of time and an Input/Output (“I/O”) size todetermine the active cache miss rate.
 3. The computer program product ofclaim 2, wherein the gathering information on the cache demote ratecomprises: for each of the time intervals gathering an amount of dataper unit of time that was removed from the active cache, wherein thecache demote rate is based on the amount of data removed from the activecache during the time intervals.
 4. The computer program product ofclaim 1, wherein the active cache miss rate comprises a peak activecache miss rate comprising a highest active cache miss rate thatoccurred during time intervals over which the active cache miss rate wasmeasured.
 5. The computer program product of claim 1, wherein thegathering the information further comprises: indicating a track demotedfrom the active cache in a ghost cache list in response to demoting atrack in the active cache, wherein the demoted track is not indicated inthe active cache list; gathering information on a ghost cache miss ratebased on a rate over time of access to tracks that are not indicated inthe active cache list or the ghost cache list; determining whether theghost cache miss rate is less than the cache demote rate in response todetermining that the active cache miss rate exceeds the cache demoterate; and in response to determining that the ghost cache miss rate isless than the cache demote rate, the additional memory space comprisesan amount of memory space associated with the ghost cache list.
 6. Thecomputer program product of claim 5, wherein the additional memory spacecorresponds to memory space in the computer system that is not yet madeavailable for use in the active cache, wherein the message to the userindicates to add to the active cache the additional memory space in thecomputer system not yet made available for use, wherein the operationsfurther comprise: receiving an instruction to add the additional memoryspace to the active cache in response to the user accepting to add theadditional memory space; and adding the memory space in the computersystem to the active cache to expand a size of the active cache and asize of the active cache list in response to the receiving theinstruction.
 7. The computer program product of claim 5, wherein thetracks indicated in the ghost cache list are not stored in the activecache.
 8. The computer program product of claim 1, wherein there are aplurality of N ghost cache lists ordered from 1 to N, wherein tracksdemoted from the active cache are indicated in ghost cache list 1, andwherein the operations further comprise: for ghost cache lists 2 throughN, indicating in the ghost cache list i a track demoted from the ghostcache list (i−1), wherein the gathered information includes ghost cachemiss rates, where ghost cache miss rate i for ghost cache list i isbased on a rate over time of accesses to tracks that are not indicatedin the active cache list or any of the ghost cache lists 1 through i;determining whether one of the ghost cache miss rates is less than thecache demote rate in response to determining that the active cache missrate exceeds the cache demote rate; and in response to determining thatone of ghost cache miss rates comprising ghost cache list i is less thanthe cache demote rate, the additional memory space comprises a totalamount of space associated with the ghost cache lists 1 through i. 9.The computer program product of claim 8, wherein determining whether oneof the ghost cache miss rates is less than the cache demote ratecomprises determining a first of the ghost cache miss rates less thanthe demote cache rate by considering ghost cache miss rates in an orderfrom the ghost cache miss rate for the ghost cache list 1 through theghost cache miss rate for the ghost cache list N, and wherein thedetermining whether one of the ghost cache miss rates is less than thecache demote rate terminates upon determining a first ghost cache list ihaving the ghost cache miss rate less than the cache demote rate. 10.The computer program product of claim 8, wherein the operations furthercomprise: in response to determining that none of the ghost cache missrates is less than the cache demote rate, the indicated additionalmemory space to add comprises indicating to add an additional computersystem.
 11. The computer program product of claim 8, wherein theoperations further comprise: demoting a track indicated in the ghostcache list 1 in response to the ghost cache list 1 being full when atrack is demoted from the cache to make an entry available in the ghostcache list 1 to indicate the track being demoted from the active cache;and for ghost cache lists 2 through N, demoting a track indicated in theghost cache list i in response to the ghost cache list i being full whena track is demoted from the ghost cache list (i−1) to make an entryavailable in the ghost cache list i to indicate the track being demotedfrom the ghost cache list (i−1).
 12. A system coupled to a storagehaving tracks, comprising: a processor; an active cache implemented inat least one memory device; and a computer readable storage mediumhaving computer program code that in response to being executed by theprocessor performs operations, the operations comprising: indicatingtracks in the active cache in an active cache list; during cachingoperations to the active cache, gathering information on an active cachemiss rate based on a rate of access to tracks that are not indicated inthe active cache list over time and a cache demote rate based on a rateof an amount of data removed from the active cache over time;determining whether the active cache miss rate exceeds the cache demoterate; determining whether adding additional memory space to the activecache would result in the active cache miss rate being less than thecache demote rate in response to determining that the active cache missrate exceeds the cache demote rate; and generating a message to a userof the system indicating to add the additional memory space to theactive cache in response to determining that adding the additionalmemory space would result in the active cache miss rate being less thanthe cache demote rate.
 13. The system of claim 12, wherein the gatheringinformation on the active cache miss rate comprises: for each timeinterval of a plurality of time intervals, indicating a number of activecache misses that occurred during the time interval; determining the anumber of the active cache misses per unit of time based on the numberof active cache misses that occurred during each of the time intervals;and multiplying the determined number of active cache misses per unit oftime and an Input/Output (“I/O”) size to determine the active cache missrate.
 14. The system of claim 12, wherein the gathering the informationfurther comprises: indicating a track demoted from the active cache in aghost cache list in response to demoting a track in the active cache,wherein the demoted track is not indicated in the active cache list;gathering information on a ghost cache miss rate based on a rate overtime of access to tracks that are not indicated in the active cache listor the ghost cache list; determining whether the ghost cache miss rateis less than the cache demote rate in response to determining that theactive cache miss rate exceeds the cache demote rate; and in response todetermining that the ghost cache miss rate is less than the cache demoterate, the additional memory space comprises an amount of memory spaceassociated with the ghost cache list.
 15. The system of claim 14,wherein the additional memory space corresponds to memory space in thesystem that is not yet made available for use in the active cache,wherein the message to the user indicates to add to the active cache theadditional memory space in the system not yet made available for use,wherein the operations further comprise: receiving an instruction to addthe additional memory space to the active cache in response to the useraccepting to add the additional memory space; and adding the memoryspace in the system to the active cache to expand a size of the activecache and a size of the active cache list in response to the receivingthe instruction.
 16. The system of claim 12, wherein there are aplurality of N ghost cache lists ordered from 1 to N, wherein tracksdemoted from the active cache are indicated in ghost cache list 1, andwherein the operations further comprise: for ghost cache lists 2 throughN, indicating in the ghost cache list i a track demoted from the ghostcache list (i−1), wherein the gathered information includes ghost cachemiss rates, where ghost cache miss rate i for ghost cache list i isbased on a rate over time of accesses to tracks that are not indicatedin the active cache list or any of the ghost cache lists 1 through i;determining whether one of the ghost cache miss rates is less than thecache demote rate in response to determining that the active cache missrate exceeds the cache demote rate; and in response to determining thatone of ghost cache miss rates comprising ghost cache list i is less thanthe cache demote rate, the additional memory space comprises a totalamount of space associated with the ghost cache lists 1 through i. 17.The system of claim 16, wherein determining whether one of the ghostcache miss rates is less than the cache demote rate comprisesdetermining a first of the ghost cache miss rates less than the demotecache rate by considering ghost cache miss rates in an order from theghost cache miss rate for the ghost cache list 1 through the ghost cachemiss rate for the ghost cache list N, and wherein the determiningwhether one of the ghost cache miss rates is less than the cache demoterate terminates upon determining a first ghost cache list i having theghost cache miss rate less than the cache demote rate.
 18. A method formanaging an active cache in a computer system to cache tracks stored ina storage, comprising: indicating tracks in the active cache in anactive cache list; during caching operations to the active cache,gathering information on an active cache miss rate based on a rate ofaccess to tracks that are not indicated in the active cache list overtime and a cache demote rate based on a rate of an amount of dataremoved from the active cache over time; determining whether the activecache miss rate exceeds the cache demote rate; determining whetheradding additional memory space to the active cache would result in theactive cache miss rate being less than the cache demote rate in responseto determining that the active cache miss rate exceeds the cache demoterate; and generating a message to a user of the computer systemindicating to add the additional memory space to the active cache inresponse to determining that adding the additional memory space wouldresult in the active cache miss rate being less than the cache demoterate.
 19. The method of claim 18, wherein the gathering information onthe active cache miss rate comprises: for each time interval of aplurality of time intervals, indicating a number of active cache missesthat occurred during the time interval; determining a number of theactive cache misses per unit of time based on the number of active cachemisses that occurred during each of the time intervals; and multiplyingthe determined number of active cache misses per unit of time and anInput/Output (“I/O”) size to determine the active cache miss rate. 20.The method of claim 18, wherein the gathering the information furthercomprises: indicating a track demoted from the active cache in a ghostcache list in response to demoting a track in the active cache, whereinthe demoted track is not indicated in the active cache list; gatheringsinformation on a ghost cache miss rate based on a rate over time ofaccess to tracks that are not indicated in the active cache list or theghost cache list; determining whether the ghost cache miss rate is lessthan the cache demote rate in response to determining that the activecache miss rate exceeds the cache demote rate; and in response todetermining that the ghost cache miss rate is less than the cache demoterate, the additional memory space comprises an amount of memory spaceassociated with the ghost cache list.
 21. The method of claim 20,wherein the additional memory space corresponds to memory space in thecomputer system that is not yet made available for use in the activecache, wherein the message to the user indicates to add to the activecache the additional memory space in the computer system not yet madeavailable for use, wherein the operations further comprise: receiving aninstruction to add the additional memory space to the active cache inresponse to the user accepting to add the additional memory space; andadding the memory space in the computer system to the active cache toexpand a size of the active cache and a size of the active cache list inresponse to the receiving the instruction.
 22. The method of claim 18,wherein there are a plurality of N ghost cache lists ordered from 1 toN, wherein tracks demoted from the active cache are indicated in ghostcache list 1, further comprising: for ghost cache lists 2 through N,indicating in the ghost cache list i a track demoted from the ghostcache list (i−1), wherein the gathered information includes ghost cachemiss rates, where ghost cache miss rate i for ghost cache list i isbased on a rate over time of accesses to tracks that are not indicatedin the active cache list or any of the ghost cache lists 1 through i;determining whether one of the ghost cache miss rates is less than thecache demote rate in response to determining that the active cache missrate exceeds the cache demote rate; and in response to determining thatone of ghost cache miss rates comprising ghost cache list i is less thanthe cache demote rate, the additional memory space comprises a totalamount of space associated with the ghost cache lists 1 through i. 23.The method of claim 22, wherein determining whether one of the ghostcache miss rates is less than the cache demote rate comprisesdetermining a first of the ghost cache miss rates less than the demotecache rate by considering ghost cache miss rates in an order from theghost cache miss rate for the ghost cache list 1 through the ghost cachemiss rate for the ghost cache list N, and wherein the determiningwhether one of the ghost cache miss rates is less than the cache demoterate terminates upon determining a first ghost cache list i having theghost cache miss rate less than the cache demote rate.